A wide variety of approaches to sealing in moderately high pressure and high-temperature environments, such as in internal combustion engines, have been used. Each has its advantages and drawbacks.
For example, in small one and two cylinder air-cooled internal combustion engines, such as those used as engines for lawn tractors and the like, compressed asbestos head gaskets have been used. These have not been entirely satisfactory, particularly where one of the engine components, such as the head, has been of aluminum. And in refrigerant compressors, the gaskets used have not always effectively sealed in certain zones, such as at cross-ribs or webs in the gasket assembly. Not only have such gaskets not been as effective as they might be, but the elevated temperature environments in which they are used have made it difficult to embrace many approaches to improving their sealing effectiveness which might have been used at lower temperatures. For example, the use of elastomeric or even silicone sealing beads is simply not feasible in head gaskets for many air-cooled engines where the temperature reaches and stays at 400.degree. F. or more.
A multitude of factors must be taken into consideration in producing an effective seal. Such factors include, among others, temperature, pressure, surface irregularities in the surfaces to be sealed, unequal torque loads on the bolts, limited available torque load, bowing of the members to be sealed and the inability to place bolts in all of those locations which would assure adequate distribution of the loading forces. Additionally, stress relaxation of many gasket materials, manufacturing tolerances and other factors inherant in the manufactured seals and gaskets themselves contribute to the difficulty of effective sealing, particularly in high temperature environments. Further, the use of a metallic gasket facilitates effective heat transfer which is especially important for air-cooled engines.